The aim of this work is to investigate the mechanisms controlling the expression of class I MHC genes. It has been demonstrated that there are 10-15 class I genes in the genome of the miniature swine. A series of genomic clones containing MHC-homologous DNA sequences has been isolated and introduced into mouse L cells. Two categories of MHC genes have been identified in this way: a set of closely related genes which are expressed in L cells and appear to represent the genes encoding the major locus products and a set of more distantly related genes which are not expressed in L cells. The regulation of expression of the swine MHC DNA segment, PD1, in mouse L cells has been used as a model system to study the regulation of expression of a single member of the multigene family. We have demonstrated that the pig DNA segment is subject to regulatory constraints indistinguishable from endogenous sequences with respect to chromatin structure, differential transcription of the segment and transcriptional enhancement by the exogenous inducer, interferon. We are now attempting to identify regulatory sequences within the gene by examining the expression capability of a series of deletion mutants. In addition, the expression vectors, pSV2cat and pSVOcat, are being used to identify any regulatory swine sequences. Differential patterns of MHC gene expression in different tissues have been examined. Preliminary data indicate that the various MHC genes display differing patterns of methylation and DNAse I sensitivity in chromatin in different tissues.